1. Field of the Invention
This invention relates to a method and an apparatus for dynamic signal switching of a merging unit in an electrical power system.
2. Description of the Related Art
Intelligent electronic devices are installed in electrical power systems to measure the voltage and current flows running through the electrical network. Said intelligent electronic devices may attempt to use these measurements in order to protect the power system against faults or abnormal oscillations, and to allow better control of the power system by human or automatic operators.
Traditionally, such measurements have been made using current and voltage transformers, transforming the power grid quantities into more manageable, safer, lower magnitude scaled quantities for input to intelligent electronic devices.
There is presently a growing trend to digitize the output of such current transformers and voltage transformers, for ease of communication and connection to intelligent electronic devices in an electrical substation automation scheme.
The same existing current transformers and voltage transformers can have their scaled, secondary side analogue signals converted into digital signals by a device termed a “Merging Unit”. The field of the present invention relates to the case where multiple current transformer inputs are available at one location, nominally offering redundancy/duplication of the same measured quantity. This is often the case where a current transformer core for protection applications and another core for measurement applications are available.
As the measurement core (measurement accuracy class) is more accurate for load currents, said measurement core would normally be considered the truest representation of the real primary current quantity on the power system. As to higher level fault current flows, the measurement current transformer core may saturate, making its output an erroneous representation of the real primary input. In such instances, the protection current transformer core having a larger dynamic range (and better immunity to saturation) would be a better scaled representation of the primary quantity.
Historically, the secondary wired current transformer circuits would have needed to run cross-site, from the primary power system equipment in the electrical substation yard, to the physical substation building where the intelligent electronic device would have been situated.
This cross-site run of current transformer circuits is potentially dangerous, as an inadvertent open circuit could yield dangerous voltages and sparking/arcing in the vicinity of the break. Hence, it is desirable to site the merging unit close to the current transformer secondary circuit which the former is digitizing.
We will then consider prior art documents.
U.S. Pat. No. 6,954,704 describes a digital protection and control device. Indeed, in conventional protection and control systems, since analog information transmitted through electric cables is used for information transmission between substation main equipments and protection and control devices that protect and control these substation main equipments, contact input circuits and contact output circuits handling a relatively large voltage and current have been required. Further, a space for disposing a large number of electric cables is necessary, and a protection unit and a control unit need to be accommodated in independent cases provided exclusively for the respective units, which has been a cause of the increase in installation space of the devices. U.S. Pat. No. 6,954,704 provides a digital protection and control device configured to realize reduction in installation space thereof by the digitalization of the device, and to realize improvement in protection and control performance by sufficient data exchange in the device. Therefore, U.S. Pat. No. 6,954,704 describes the basic principles of a merging unit.
EP 1 845 383 describes a method of detecting saturation of a current transformer. Said method involves detecting a fault of a secondary current/voltage waveform of a current transformer. A magnetic flux in the secondary of the transformer is estimated by integration of the secondary current during a time window from the detection of the fault. The estimated magnetic flux is compared with a threshold value, where saturation of the transformer is detected while the threshold value exceeds the magnetic flux.
There is no existing solution to allow the input from duplicate (or multiple) measurement sources to combine as a single “optimized” one.
The only way this could be achieved today is to have two separate merging units, each digitizing only one 3-phase set of current transformer signals. The two merging units would then multicast two separate measurements of sampled values, which could be connected to the same Ethernet link. Hypothetically, any intelligent electronic devices could be configured so as to be capable to read the two (or more) separate sources concurrently, and choose between them according to quality bits which are available in the input signals. However, no intelligent electronic device on the market today can offer such capability.